An electronic device, such as a memory device, includes on-chip circuits, such as memory circuits. The electronic device also includes terminal contacts for electrically connecting the on-chip circuits to external circuits, such as other components of a memory system. Physically, the terminal contacts can comprise balls, pins or leads formed on the outside of a semiconductor package containing the electronic device. Electrically, the terminal contacts are referred to as pins. In a memory device, the pins can include data (DQ) pins, address pins, and control pins, all of which are electrically connected to on-chip conductors (e.g., data lines, address lines, control lines). The pins transmit signals between the on-chip circuits on the memory device, and external circuits on another device, such as a processor or a controller.
In addition to memory circuits, the on-chip circuits in a memory device can include circuits for transmitting signals through the pins. For example, an input/output circuit can include output drivers configured to receive data bits from the memory circuits, and to transmit data signals from the memory circuits to the pins. The on-chip circuits can also include circuits for improving the electrical characteristics of the signals transmitted through the pins.
One electrical characteristic of the pins that can be controlled is the pin impedance. For example, an impedance calibration circuit can be used to tune various transistors in output drivers of the input/output circuit for selected pins of the memory device. The impedance calibration circuit can operate in concert with an ODT (on die termination) circuit to control on die termination impedances. Using these circuits, the impedance of one or more pins can be adjusted as a function of the impedance of an associated device such as a microprocessor (MPU). However, operation of these circuits also affects pin capacitance in the memory device.
Semiconductor manufacturers typically have pin capacitance specifications that insure the integrity of the signals transmitted through the pins. In general, high pin capacitance adversely affects signal integrity due to reflections. This is a particular problem at high speeds where impedance mismatches on signal transmission lines cause reflections. High pin capacitance also adversely affects system power, and slows down the edge rate of the signals, which reduces the data valid window. Meeting the tight pin capacitance specifications is becoming more challenging on current generation high speed memory devices, particularly at fast speed grades. For example, double data rate (DDR) memory devices, such as DDR3 memory devices manufactured by Micron Technology Inc. of Boise, Id., support data rates of 800-1600 Mbits/s and clock frequencies of 400 to 800 MHz. In these high speed memory devices, it is desirable to keep pin capacitance low, so that transmission signals can be efficiently processed by electronic elements receiving the signals.
In the art, various methods and circuits have been developed for controlling pin capacitance in electronic devices. For example, U.S. Pat. Nos. 7,151,700 and 7,164,600 to Ba describe a method and circuit for reducing pin capacitance in a memory device. With this method tuning transistors in ODT circuits are turned off during a default operational state (e.g., no data read operation occurring) to reduce DQ pin capacitance.
The present disclosure is directed to a method and circuit for controlling pin capacitance in an electronic device, without affecting the speed of the signals transmitted through the pins. In addition, by reducing DQ pin capacitance the present method and circuit allow circuits which share DQ pads to use more of the allowed pin capacitance budget, thereby permitting more robust designs, such as improved ESD (electrostatic discharge) circuits.